This invention relates generally to nuclear reactor fuel assemblies and more particularly to providing a wear sleeve in the control rod guide tubes of the assembly whereby vibration of the control rod will not damage the guide tube.
Nuclear reactors of the pressurized water type typically have a core region consisting of a multiplicity of vertically oriented fuel assemblies, each assembly containing a matrix of fuel pins. The assembly skeleton includes a plurality of elongated guide tubes to which are connected grids for supporting the fuel pins, and end fittings for securing the assembly between vertically spaced support plates. The guide tubes also serve as sheaths for control rods which are inserted into the core for the purpose of controlling the heat output of the fuel. An upward flow of liquid is maintained in the guide tube to cool the control rod.
Examination of selected fuel assemblies during the refueling of some reactors of this type has revealed the existence of wear patterns on the inside of the guide tubes at the elevation corresponding to the position of the control rod tip within the guide tube when the rod is in the upper limit of travel, i.e., the unique "withdrawn" position. Such wear behavior has the potential for perforating the guide tubes and weakening them so much that the integrity of the assembly might be in doubt. Significant weakening of the guide tubes is particularly dangerous during refueling when the full weight of the assembly is borne by the guide tubes. Thus, further wear of guide tubes in assemblies that are present in a particular reactor must be prevented, and new assemblies that have been fabricated but not yet loaded into the reactor should be modified to avoid such wear.
Modifying fuel assemblies after they have been in the reactor, or when stored on site prior to placement in the reactor, is a difficult task because the assemblies are typically kept under water. Thus, the modification must be made remotely. Accordingly, the problem addressed by the present invention preferably is solved in a way that is easily adopted for remote modification of worn guide tubes.